1. Field
The present invention relates to an acoustic transducer that detects sound waves using change in the capacitance between a vibrating electrode and a fixed electrode, converts the sound waves into an electrical signal, and outputs the electrical signal.
2. Related Art
Conventionally, an ECM (Electret Condenser Microphone) that employs an electret has been widely used as a small-size microphone for installation in a mobile phone device or the like. However, since charge retention in an electret is easily influenced by a hot atmosphere during microphone manufacturing, there have been cases where maintaining sufficient quality is difficult. This has led to an understanding of the superiority of MEMS microphones, which employ capacitor-type acoustic transducers that detect sound waves and convert them into an electrical signal (detection signal). Note that this acoustic transducer is also called a MEMS microphone since it is manufactured using MEMS technology.
The wider the compatible sound pressure range (dynamic range) of detectable sound waves is in a microphone, the more convenient and useful the microphone is. The dynamic range is defined by the highest compatible sound pressure (acoustic overload point, which is referred to hereinafter as the “AOP”), which is determined by the harmonic distortion rate (total harmonic distortion, which is referred to hereinafter as the “THD”), and the lowest compatible sound pressure, which is determined by the signal-to-noise ratio. When a microphone attempts to detect a sound having a high sound pressure in the vicinity of the highest compatible sound pressure, harmonic distortion occurs in the output signal, and the sound quality deteriorates. In view of this, for example, JP 2012-147115A discloses technology in which, in an acoustic transducer having a fixed electrode and a vibrating electrode that oppose each other, the main portion of the vibrating electrode is divided by a slit so as to divide the capacitor structure in the acoustic transducer into a high-sensitivity variable capacitor and a low-sensitivity variable capacitor, thus widening the dynamic range of the microphone.
Also, as technology for improving the signal-to-noise ratio and lowering the lowest compatible sound pressure, JP 5049312B discloses technology in which a floating type of vibrating electrode is configured in an acoustic transducer for a MEMS microphone. In this technology, the vibrating electrode is arranged in a free state when the microphone is not being used since a voltage is not applied to the vibrating electrode and the fixed electrode, but when a voltage is applied to the two electrodes to perform sound wave detection, electrical attraction occurs between the electrodes, and the vibrating electrode becomes positioned and fixed against the fixed film in a state in which the vibrating electrode abuts against protrusion portions provided on the fixed film side. This improves the sensitivity of the microphone, which as a result enables realizing favorable acoustic characteristics with a good signal-to-noise ratio.
JP 2012-147115A and JP 5049312B are examples of background art.